1) Field of the Invention
The present invention relates to a broadband pulse width modulation circuit suitable for use with a constant current driving source which utilizes a pulse width modulation technique used, for example, for an optical amplifier, an optical communication apparatus and so forth and an optical amplifier using the broadband pulse width modulation circuit.
2) Description of the Related Art
FIG. 8 is a block diagram showing a configuration of an existing optical amplifier. The optical amplifier shown in FIG. 8 branches part of input and output light of, for example, an erbium-doped optical fiber (EDF) 100 as monitor light by means of optical couplers 200 and 400 and supplies the branched light to light reception devices 500 and 600 such as PIN photodiodes, respectively. Then, the optical amplifier monitors the levels of the input and output lights of the EDF 100 to detect an error therebetween by means of an error amplifier 700, and performs proportional plus integral plus derivative (PID) control of an output (excitation light power) of an excitation light laser 1000 such as semiconductor laser by means of a PID controller 800 so that a predetermined amplification gain is obtained based on a result of the detection. It is to be noted that the excitation light is supplied to the EDF 100 by an optical coupler 300 provided on the input side of the EDF 100.
Here, in order to drive the excitation light laser 1000, generally high current (several hundred mA or more) is required, and therefore, reduction of the power consumption is demanded. To this end, it has been proposed to use a pulse width modulation circuit (PWM circuit) 900 in which a field effect transistor (FET) or a bipolar transistor is used as a switching device as a circuit technique for reducing the power consumption. The PWM circuit is utilized also in order to reduce the power consumption of components used in an optical communication apparatus and requiring high current such as a semiconductor laser, an electronic cooling device, an electric heater and so forth.
It is to be noted that a technique which uses a PWM circuit is disclosed, for example, in Japanese Patent Laid-Open No. HEI 5-333953 (hereinafter referred to as Patent Document 1) or Japanese Patent Laid-Open No. HEI 7-326055 (hereinafter referred to as Patent Document 2).
The technique disclosed in Patent Document 1 is applied to a static reactive power compensation apparatus incorporated in an electric system, to which a fluctuating load is applied such as that of an electric vehicle to suppress the fluctuation of a system voltage. In the static reactive power compensation apparatus, when an instantaneous voltage drop is caused by closing of an AT feeder or changeover closing of an electric car, PWM high-speed response control is suitably applied in response to CB commission information in order to reduce the total driving loss upon inverter driving. To this end, a PWM carrier frequency changeover circuit is provided to a PWM gate control circuit for the inverter so that a PWM carrier frequency can be changed over. Thus, with the static reactive power compensation apparatus, during steady operation, a rectangular wave is outputted to reduce the switching loss, and only a voltage fluctuation caused by rush current upon transient response (such as changeover closing of an electric car) is controlled at a high speed by the PWM control to suppress an instantaneous drop of the voltage. Further, when the steady state is restored, the rectangular wave output is restored to reduce the switching loss. Accordingly, by suppressing the transient voltage fluctuation and besides reducing the total driving loss, inverter driving can be performed with a high efficiency.
On the other hand, the technique disclosed in Patent Document 2 is directed to a method of controlling a laser writing output level in an optical disk recording apparatus applied to a data recording apparatus for a write-once-read-many optical disk for automatically calibrating a laser using a writing pulse focused on the disk during write data operation. Also use of a PWM signal for a testing signal of a writing pulse for data recording is disclosed in Patent Document 2.
However, where the PWM circuit 900 is used to control the excitation light laser 1000 of the optical amplifier as described above with reference to FIG. 8, a PWM output pulse must be smoothed and a coil (L) and a capacitor (C) for smoothing must be required. Therefore, there is a defect that a signal frequency band is limited by the time constant of the LC elements. In recent years, for an optical communication apparatus, particularly in optical amplifiers, high speed responsibility is required. However, where a PWM circuit which requires a smoothing circuit having LC elements in this manner is used, since a signal frequency band is limited significantly, there is a subject that satisfaction of the requirement for a high speed response is not easy. Further, even if any one of the techniques disclosed in Patent Documents 1 and 2 is applied, the subject just described is not solved.